Frequently, excessive voltage is applied across service lines that deliver power to residences and commercial and institutional facilities. Such excess voltage or voltage spikes may result from lightning strikes, for example. The voltage surges are of particular concern in telecommunications distribution centers, hospitals and other facilities where equipment damage caused by voltage surges and resulting down time may be very costly.
Typically, one or more varistors (i.e., voltage dependent resistors) are used to protect a facility from voltage surges. Generally, the varistor is connected directly across an AC input and in parallel with the protected circuit. The varistor has a characteristic clamping voltage such that, responsive to a voltage increase beyond a prescribed voltage, the varistor forms a low resistance shunt path for the overvoltage current that reduces the potential for damage to the sensitive components. Typically, a line fuse may be provided in the protective circuit and this line fuse may be blown or weakened by the surge current or the failure of the varistor element.
Varistors have been constructed according to several designs for different applications. For heavy-duty applications (e.g., surge current capability in the range of from about 60 to 200 kA) such as protection of telecommunications facilities, block varistors are commonly employed. A block varistor typically includes a disk-shaped varistor element potted in an epoxy or plastic housing. The varistor disk is formed by pressure casting a metal oxide material, such as zinc oxide, or other suitable material such as silicon carbide. Copper, or other electrically conductive material, is flame sprayed onto the opposed surfaces of the disk. Ring-shaped electrodes are bonded to the coated opposed surfaces and the disk and electrode assembly is enclosed within the plastic housing. Examples of such block varistors include Product No. SIOV-B860K250, available from Siemens Matsushita Components GmbH & Co. KG and Product No. V271BA60, available from Harris Corporation.
Another varistor design includes a high-energy varistor disk housed in a disk diode case. The diode case has opposed electrode plates and the varistor disk is positioned therebetween. One or both of the electrodes include a spring member disposed between the electrode plate and the varistor disk to hold the varistor disk in place. The spring member or members provide only a relatively small area of contact with the varistor disk.
Another type of overvoltage protection device employing a varistor wafer is the Strikesorb™ surge protection module available from Raycap Corporation of Greece, which may form a part of a Rayvoss™ transient voltage surge suppression system. (See, for example, U.S. Pat. No. 6,038,119, U.S. Pat. No. 6,430,020 and U.S. Pat. No. 7,433,169.
Varistor-based overvoltage protection devices (e.g., of the epoxy-shielded type) are commonly designed with an open circuit failure mode using an internal thermal disconnector or overcurrent disconnector to disconnect the device in case of failure. Other varistor-based overvoltage protection devices have a short circuit as a failure mode. For example, some epoxy-shielded devices use a thermal disconnector to switch to a short circuit path. However, many of these devices have very limited short circuit current withstand capabilities.